The present invention relates to means for connection between a rod and crosshead in a reciprocating compressor.
Priority is claimed to Italian patent application MI 2001/A00/1757, filed Aug. 9, 2001.
The most common type of reciprocating compressors consist of one or a plurality of cylinders inside which pistons provided with reciprocating motion slide.
As far as the kinematic control mechanism for the reciprocating motion of the piston is concerned, reference is made by way of example to the kinematic mechanisms which consist of a connecting rod and crank or a connecting rod, crank and crosshead.
The second type of transfer of reciprocating motion proposed is applied to machines which do not have a direct connection between the foot of the connecting rod and the piston, but require interposition of a stem or rod.
However, the rod is not designed to withstand transverse forces and it is therefore necessary to provide at its articulation with the foot of the piston a straight guide which forces the center of the articulation to move along the axis of the cylinder, constituted by the so-called crosshead.
The crosshead consists of a body provided with a journal or pin for articulation with the connecting rod, and with runners, wherein the axis of the pin is at right-angles to the axis of the rod and of the runners, i.e. it is disposed in the shape of a cross relative to the latter.
In general the runners are produced with a cylindrical surface made of cast iron or steel or light alloy, are integral with the body or added onto the latter and are covered with anti-friction metal. The runners slide in guides with a complementary cylindrical surface.
In general the body is produced from cast iron, cast steel or forged steel.
Hereinafter for the crosshead reference will be made by way of example to the configuration with a pin and two runners, which is designed for connection to drive shafts which can rotate in both directions, thus exchanging transverse forces with both the guides.
At present the means for connection between the piston rod and the crosshead in a reciprocating compressor consist of common threaded units, i.e. they consist of a nut and a single lock nut.
For correct fitting use is also made of elements to stop rotation, of the key type.
In the fitting solutions generally used the means for connection between the rod and the crosshead, i.e. the nut and lock nut, abut supports, for example flanges, which are finally connected to the crosshead by means of tie rods.
The tightening of the threaded units, such as to apply a predetermined load which assures the hold of these means for connection in use can be carried out manually for example, in the case of reciprocating compressors with low performance, or hydraulically.
The hydraulic tightening in the case of high-performance machines which must guarantee maximum reliability and safety makes it possible to obtain better preloading and thus better hold of the connection between the rod and the crosshead.
The hydraulic tightening of the means for connection between the rod and crosshead in reciprocating compressors is carried out for example by positioning the hydraulic jack on the rod, in the section between the nut and the lock nut, such as to abut between the nut and the support, which in turn is secured to the crosshead.
This solution is structurally more complicated, since corresponding holding means must be provided for the correct functioning of the hydraulic jack.
The main disadvantage of the means for connection between the rod and crosshead in a reciprocating compressor such as those used at present relates to limitation of the load applicable to the rod during functioning. On the basis of the foregoing information this value is dependent on the nominal value of the preloading applied during fitting and above all on the extent of the relaxation once the tightening has been completed, which is expressed in the form of a relaxation coefficient.
In fact, in the present fitting solutions, in a reciprocating compressor the means for connection between the rod and the crosshead, i.e. the nut or lock nut, are not subjected to stress during the fitting step but only at the moment of release of the traction load applied in order to connect the rod to the crosshead. The coefficient of relaxation is therefore not optimized with these fitting solutions.
In reciprocating compressors the means for connection between the rod and crosshead constitute a critical characteristic for the reliability and performance of the machine.
The dynamic stresses induced by the cyclical load, which is characteristic of the functioning of reciprocating compressors, can limit the performance of these means for connection above all from the point of view of fatigue strength, which is characteristic of the reciprocating motion functioning system.
The means for connection between the rod and crosshead of a reciprocating compressor used hitherto do not however constitute an optimized response in terms of performance of the machine.
A means is disclosed for connection between the rod and crosshead in a reciprocating compressor which eliminate the above-described disadvantages.
A means is disclosed for connection between the rod and crosshead in a reciprocating compressor which make it possible to obtain higher performance levels in terms of loads which can be applied to the rod for the same nominal preloading and type of thread of the nuts.
A means is disclosed for connection between the rod and crosshead in a reciprocating compressor which are particularly simple and functional, with low costs.